1. Field of the Invention
The present invention relates generally to displays and, in some embodiments, to displays having a transistor-degradation circuit.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Liquid-crystal displays (LCDs) are used in a variety of electronic devices, such as televisions, computer monitors for desktop and laptop computers, and specialized equipment like automated teller machines, medical devices, and industrial equipment. LCD panels are also used frequently in portable electronic devices, such as cell phones, global-positioning-satellite (GPS) units, and hand-held media players.
Typically, LCD panels include an array of pixels for displaying images. The pixels often each include three or more sub-pixels that each display a color, e.g., red, blue, green, and in some instances, white light. To display an image, the appropriate sub-pixels on the display are rendered transmissive to light, allowing color-filtered light to pass through each of the transmissive sub-pixels and form the image. The sub-pixels are often arranged in a grid and can be addressed, e.g., individually adjusted, according to their row and column in the grid. Generally, each sub-pixel includes a transistor that is controlled according to row and column signals. For instance, the gate of a transistor in a sub-pixel may connect to a gate line generally extending in the column direction, and a source of the transistor in the sub-pixel may connect to a source line generally extending in the row direction. Often, a plurality of the transistors in the same column have gates connected to the same gate line, and a plurality of the transistors in the same row have sources connected to the same source line. An individual sub-pixel is typically addressed by turning on its transistor through the gate line, and transmitting image data relevant to the individual sub-pixel through its source line. By repeating this addressing process for each of the pixels in the display, an image may be formed, and by sequentially displaying changing images, video may be displayed.
Some components of LCD panels perform differently as the LCD panel ages. Each of the gate lines is often controlled by a number of gate-line transistors disposed at one end of the gate line. Typically, at least one gate-line transistor, having a high duty cycle, is employed to pull the gate line down, as will be described further below. Generally, the gate-line transistor is disposed in series between the transistors in the sub-pixels and a voltage source that tends to turn off the transistors in the sub-pixels. Accordingly, the gate-line transistor is typically in a conductive state except when its associated sub-pixels are being addressed, as the transistors of non-addressed sub-pixels are typically left in an off state to preserve the light-transmitting state of the sub-pixels. When the LCD panel is operating, a given column of sub-pixels is addressed relatively infrequently, as LCD panels often include a large number, e.g., several hundred or several thousand, columns of sub-pixels, and one column of sub-pixels (or some other subset) is addressed at a time. As a result, in some LCD panels, the gate-line transistors spend a substantial portion of the panel's life in a conductive state, holding the transistors on their gate line in an off state. This high duty cycle often results in the properties of the gate-line transistors changing during the life of the panel. For instance, the threshold voltage of the gate-line transistors may increase over the life of the panel.
The rate of change, however, is difficult to predict. Thermal variations across the display may affect the rate of change in the threshold voltage, and process variations during the manufacture of the display may affect the rate of change in the threshold voltage. Consequently, it has proven difficult to estimate the change in the threshold voltage of the gate-line transistors.